Materials with pozzolanic properties contain glassy silica and alumina that will, in the presence of water and free lime, react with the calcium in the lime to produce calcium silicate hydrates. There are both natural and industrial pozzolans.
Industrial pozzolans are produced during industrial processes and include, for example, Class C and Class F fly-ashes. These fly-ashes are produced during the combustion of coal. They consist of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphous structure. The classification of industrial produced fly-ash depends on the type of coal used. The combustion of lignite or subbituminous coal typically produces Class C fly-ash. The combustion of anthracite and bituminous coal typically produces Class F fly-ash.
As defined by ASTM C618, the characteristics of Class C fly-ash include higher lime content than Class F fly-ash. The higher lime content in Class C fly-ash allows Class C fly-ash to be self-hardening, while Class F fly-ash typically requires the addition of additional lime or cement to form cementitious materials.
An example of a natural pozzolan is Class N pozzolanic ash. Class N pozzolanic ash is a raw or calcined natural pozzolan such as some diatomaceous earths, opaline cherts, and shales; tuffs, volcanic ashes, and pumicites; and calcined clays and shales.
The chemical and physical attributes of Class C, Class F and Class N pozzolanic ashes are further defined by ASTM-C618, the contents of which are hereby incorporated by reference in their entirety.
It is known to use fly-ash in cement compositions, typically in the presence of Portland cement. U.S. Pat. No. 5,556,458 to Brook, et al., for example, requires at least 20% Portland cement. In particular, Brook, et al. notes that Portland cement is required in their composition to overcome the low early strength of fly ash compositions.
U.S. Pat. No. 4,997,484 to Gravitt et al. and U.S. Pat. No. 7,288,148 to Hicks et al. disclose fly-ash cement compositions without Portland cement, but rely on an acid-base reaction system that utilize the combined effects of citric acid (≈pH 2.2) and either an alkali hydroxide (≈pH 12-14) or metal carbonate (≈pH 11.6). However, the fly-ash cement compositions of Gravitt and Hicks do not exhibit the strength and hardness required by many cement applications.